During cartilage degeneration associated with osteoarthritis, chondrocyte proliferation (cloning), hypertrophy, and calcification occur, resembling a recapitulation of the events which occur during endochondral bone formation. Recently a number of new genes which are expressed during chondrocyte maturation have been characterized, including PTHrP, PTH/PTHrP receptor, indian hedgehog (IHH), annexin V, and BMP6. We have also identified NHE1 (a Na+/H+ exchanger isoform) as a candidate molecule under regulation of PTHrP and BMP6 which drives the chondrocyte hypertrophic volume increase. We have identified PTHrP and BMP6, which are not expressed in normal adult articular cartilage, in human OA cartilage. This has led to our overall hypothesis that cytokine production in OA leads to activation of the chondrocyte maturation pathway. Elements of this pathway, including cell proliferation, MMP production, hypertrophy, matrix turnover, and apoptosis, may contribute to the pathophysiology of OA. Understanding the regulation of the maturational pathway may therefore lead to new diagnostic markers or therapeutic targets in OA. Specific Aim 1 will use well characterized in vitro chondrocyte culture models to study the role of IHH, PTHrP, and the PRHrP receptor in the initiation of maturation. Specific Aim 2 will examine the effect of TNF and other cytokines on gene expression associated with chondrctye maturation. Specific Aim 3 will correlate the in vitro findings with tissue-based studies of chondrocyte maturation using a TNF overexpression murine arthritis model, and human OA cartilage. Thus, this proposal will examine the inter-relationships between BMPs, PTHrP and their coordinate role in the regulation of chondrocyte maturation during endochondral ossification.